Synthesis of a novel alkaline-developable photosensitive copolymer based on MMA, MAA, SM, and 2-HEMA-grafted GMA copolymer for an innovative photo-imageable dry-peelable temporary protective plastisol
Methyl methacrylate, methacrylic acid, styrene and 2-hydroethyl methacrylate were used to synthesize copolymer with carboxylic acid group through free-radical polymerization to develop a prepolymer(PMMSH). Glycidyl methacrylate was then added to react with carboxylic acid groups of PMMSH to form photosensitive copolymer PMMSHgG. Nuclear magnetic resonance and infrared were used to confirm the structures and functional group of the prepolymer and the copolymer while GPC was employed to determine the molecular weight. The sensitivity and resolution of the copolymers were obtained through characteristic exposure curve and SEM observation, respectively. PMMSHgG was also mixed with dry-peelable plastisol, acrylic acid monomer, and photoinitiator to form a photo-imageable dry-peelable plastisol. A suitable peelable composition without scum after UV exposure and post-baking was identified.
KeywordsDry-peelable plastisol Photo-imageable resist Photosensitive copolymer Polyvinyl chloride (PVC) Free-radical polymerization
Temporary protective coatings are widely used to protect the surfaces of various products, parts, or components during their manufacture, assembly, and shipping [1, 2, 3, 4, 5]. These coatings are used to prevent surface damage such as scratching, staining, and also used to simplify or enhance cleaning operation. A few water soluble examples include water-based polyurethane dispersions [6, 7], water-based vinyl-acrylic and acrylic copolymer emulsions . Those protective films are subsequently removed by dissolution in water. For traditional photo-etching process with negative photoresists for manufacturing indium tin oxide(ITO) touch panel, strong base solution is commonly used to remove the photoresist films after etching process, but this treatment might damage ITO and further decrease the electric conductivity. Therefore, removed protective film simply by physical force (i.e., peeling by hand or adhesive tape) is an attractive strategy to solve the above-mentioned problem. The effective approach is the use of dry-peelable films such as polyvinyl acetate based strippable polymer  for radioactive decontamination, waterborne self-crosslinkable sulfourethane-silanol dispersion , and poly(vinyl alcohol) film . Dry-peelable plastisol based on polyvinyl chloride [12, 13] which is resistant to high temperature is also a type of temporary protective coating. For example, peelable solder mask  is used in the manufacture of PCB when temporary protection is needed on certain areas, especially during high-temperature process, such as hot-air leveling or plating for resistance to strong acid and alkaline. However, all these dry-peelable temporary protective coating cannot protect micro areas. The application scope will be expanded if photoresist [15, 16] resin and peelable plastisol  could be combined. This current study described a new photoresist copolymer  combined with polyvinyl chloride resin to form a photo-imageable dry-peelabe temporary protective plastisol.
Photoresists play an important role in microminiaturization in the electronic industry [18, 19, 20, 21], such as integrated circuit (IC) photoresist developed for copper wiring process  and microcircuits for printed circuit boards . They are also used for the preparation of three primary colors (R, G, and B) and black pixels in color filters of LCD [24, 25, 26]. Negative dry-film photoresist assembly comprises acrylic photosensitive copolymer [27, 28] with carboxylic acid groups, monomer, and photoinitiator. It is covered by negative with images and goes through UV exposure and development with aqueous solution of weak alkali sodium carbonate (Na2CO3). After the plating or etching process and delivery of protection function, the photoresist will be removed via soaking in strong alkali, such as NaOH(aq).
The molecular weight of the copolymer would affect the operability and ability of the developing process. Oligomer (2-hydroxy-ethyl methacrylate-co-cyclohexyl methacrylate-co-isobonyl methacrylate-co-acrylic acid) acquired through free-radical polymerization via chain transfer (Mn = 1,677) was used as the aqueous base developable negative photoresist . Polymer prepared in current study was poly (methyl-methacrylate-co-methacrylic acid-co-styrene-co-2-Hydroethyl methacrylate-grafted-glycidyl methacrylate) (PMMSHgG), which was synthesized through free-radical polymerization but having a large molecular weight. Since photoresist polymer that is subject to contact exposure must have a large molecular weight. The other merit of this polymer is that it is compatible with poly(vinyl chloride), a major component in peelable compositions .
In this study, photoresist copolymer(PMMSHgG) was synthesized and mixed with polyvinyl chloride resin, plasticizer, photoinitiator, and acrylate oligomer to form a photoresist formulation. Solid-state membrane was obtained through coating, pre-baking, and removal of solvent. The sample was then covered by a negative with images and went through UV exposure, developed via weak alkaline and post-baking. The process will smelt the polymer, plasticizer, and polyvinyl chloride resin(PVC) to form peelable membrane. In component production, the negative photoresist turned into a tough peelable membrane and had the protective function against scratching, dust, high temperature, strong acid, and strong alkaline. After the temporary protection function was over, the membrane could be peeled through physical method, which would prevent the component from damages by acidic or alkaline chemicals and provide temporary protection on precision areas.
Methyl methacrylate(MMA), methacrylic acid(MAA), styrene, 2-Hydroethyl methacrylate(HEMA) and hydroquinone monomethylether(HQME) were purchased from Showa chemical company. Glycidyl methacrylate(GMA) was obtained from SIGMA. All materials were used as received. Trimethyolopropan triacrylate, dipentaerythritol hexaacrylate, epoxy acrylate oligomer(9,770), aromatic urethane acrylate oligomer(670A2), and diallyl isophthalate prepolymer(DAP) were used as monomers in the resist formulation. PVC POWERS(PR450, PR1069) were provided from Formosa Plastic and PVC resin(H11/59) was purchased from Vinnol.
The IR spectra were recorded on a Perkin Elemer 842 spectrometer with a KBr plate. The 1H NMR spectra were recorded on a Bruker MSL 500 MHz NMR spectrometer in d-DMSO. Number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity of copolymer were estimated from gel permeation chromatography (GPC) with TSK columns (AW2500*3) and polystyrene was used as the standard. The acid number in the photosensitive copolymer was analyzed via titration with KOH.
Synthesis of Poly(MMA-co-MAA-co-styrene-co-HEMA)(PMMSH)
Synthesis of PMMSHgG
Preparation of PMMSH-based and PMMSHgG-based photoresist formulation and lithographic evaluation
Formulation of photoresist
Photoinitiator (Irgacure 907)
Monomer (trimethylolpropane triacrylate)
Solvent (diethylene glycol monoethyl ether acetate)
foam control agent
Preparation of PMMSHgG-based photo-imageable dry-peelable plastisol formulation
Formulation of photo-imageable dry-peelable plastisol
foam control agent
(B)Physical property tests
A glass substrate with dimension of 20 cm × 30 cm was prepared. It was washed with pure water and wiped with acetone, then dried in a oven with circulation of hot air at 60 °C for 15 min. Photoresist peelable plastisol was prepared with the formulation shown in Table 2. Three different types of PVC powder including PR-450, PR-1069, and H 11/59 were tested. After 10 min of stirring, the sample was placed in a three-roller mill and grinded twice. The dried and cooled glass was then placed on a horizontal coating machine. The photoresist peelable plastisol was coated with a 50 um scraper horizontally then stand still for 3 min. The glass substrate was dried in a oven with the circulation of hot air at 60 °C for 30 min and finally was placed into the exposure machine, vacuumed, and started exposure until the energy accumulated to 400 mJ/cm2. After completing exposure, the glass substrate was placed into a oven with circulation of hot air (130 °C) for 30 min. After cooling, the membrane was peeled directly and was observed whether it could be peeled completely, thus, a peeling test and tensile damage test were conducted.
Results and discussion
Synthesis and characterization of PMMSH
Relationship between composition and reaction time of PMMSH(Reaction temperature: 80 °C)
Relationship between PMMSH Reaction Temperature and Molecular Weight
The acid value decides the rate for the developing process. The content of MAA segments in copolymer decides the acid value and makes it soluble in alkaline developing solution. For environmental protection purpose, the process has to avoid the use of organic developing solution. The acid value of copolymer(PMMSH) was measured three times and the average value was 52.58 mg-KOH/g.
Synthesis and characterization of PMMSHgG
Relationship between reaction time and molecular weight of PMMSHgG(Reaction temperature: 80 °C)
Relationship between reaction time and acid value of PMMSHgG(Reaction temperature: 80 °C)
Lithographic evaluation of PMMSH-based and PMMSHgG-based Photoresists
Lithographic evaluation of PMMSHgG-based Photo-imageable Dry-Peelable plastisol on CCL (Copper Clad Laminate)
Peelable evaluation of PMMSHgG-based photo-imageable Dry-peelable plastisol
Peelable Test of PMMSHgG. The PVC used were (a) PR-450, (b)PR-1069, and (c) H 11/59
65 ± 2
78 ± 2
59 ± 1
In summary, the results of our study show the effectiveness of our approaches to obtain photosensitive prepolymer (PMMSH) via free-radical copolymerization of monomers, including, MMA, MAA, SM and HEMA. Then carboxylic acid groups on MAA are used to graft GMA. Some acid groups are kept as the developing enhancer. Molecular weight of PMMSH decreases with raising reaction temperature. Grafting GMA on PMMSH could form a photosensitive copolymer(PMMSHgG) with good sensitivity. Precision images of both photoresists PMMSH and PMMSHgG could be obtained through exposure and development.
Photosensitive copolymer PMMSHgG and poly(vinyl chloride/vinyl acetate)(H11/59) are compatible, since they both have similar ester moiety. After post-baking, plasitisol (H11/59) was plasticized into tough membrane, which was physically peelable and had no scum left after peeling. In summary, combination of photosensitive copolymer, PMMSHgG and peelable poly(vinyl chloride/vinyl acetate) could fabricate into an innovative photo-imageable dry-peelable temporary protective plastisol successfully.
The authors thank the National Science Council of the Republic of China for the partial financial support of this work under the grant no. of NSC98-2622-E-007-006-CC3.
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